1. Field of the Invention
The present invention relates to hydraulic systems that control fluid flow to a plurality of hydraulic actuators which produce movement of a combination of mechanical components on a machine, and in particular to controlling hydraulic actuators for the pivot angle and length of a machine member, such as the boom on construction equipment.
2. Description of the Related Art
Construction and agricultural machines employ hydraulic systems to operate different mechanical elements. For example, a telehandler is a common material handling machine that has a pair of forks or a platform attached to the end of a telescopic boom pivotally coupled to a tractor. Separate hydraulic actuators are employed to change the pivot angle, i.e. raise and lower the boom, and change the length of the boom, with each of those operations being referred to as a “function” of the machine. The term “hydraulic actuator”, as used herein, generically refers to any device, such as a cylinder or a motor, that converts hydraulic fluid flow into mechanical motion.
Heretofore, the machine operator controlled each function independently by manipulating the handles of different control levers. Each control lever operated a valve that governed the flow of pressurized fluid from a pump to the associated cylinder and the return flow of fluid to a tank that supplied the pump. Traditionally the control lever was mechanically linked to the valve, however there is a present trend away from mechanically operated hydraulic valves toward electrical controls using solenoid operated valves. Electrical control systems enable the valves to be located remotely from the operator cab, such as on the cylinder being controlled, for example. This simplifies the hydraulic plumbing on the machine, as a single pair of supply and tank return lines can be run in common to all the hydraulic actuators on a boom.
The operator often has to manipulate several control levers simultaneously in order to perform a given machine operation. In the case of a telehandler for example, both the boom elevation, or pivot, angle and its length must change to move the forks or platform horizontally. These simultaneous, coordinated maneuvers can be difficult to perform, especially by inexperienced operators.
Therefore, it is desirable to enable manipulation of a single control lever to control complex machine motion that involves the simultaneous, coordinated operation of a plurality of machine functions.